This invention relates to nondestructive evaluation of conductive materials to detect discontinuities such as cracks, voids and other types of flaws both at and below the surface of such materials. More particularly, this invention relates to detection methods and apparatus for inducing eddy currents into predetermined regions of a metal object and monitoring the magnetic field established by the eddy currents to provide an indication of flaws within the metal object.
It is well-known that eddy currents will be produced in an electrically conductive object that is subjected to a time-varying magnetic field and that such eddy currents establish a magnetic field that interacts with the original magnetic field in a manner which tends to oppose changes in the original field. This principle has been utilized in the prior art in a variety of manners including measurement of the conductivity of metal samples and nondestructive evaluation of conductive materials to detect internal flaws such as cracks or voids and unbonded areas of clad metal objects. In such prior art apparatus for conductivity measurement and nondestructive evaluation, a field generating coil is excited with a sinusoidally varying current or other periodic (CW) signal to produce a corresponding periodically varying magnetic field and the interaction between the magnetic field generated by the excitation signal and the magnetic field established by the eddy currents is detected to determine whether or not a conductive material is free of discontinuities. Generally, such detection is accomplished by monitoring the composite magnetic field by sensing the effect the eddy currents have on the energization signal or by sensing the signal induced in either a secondary winding of the energization coil or a separate coil that is positioned within the composite magnetic field.
Prior art CW eddy current devices exhibit several disadvantages and drawbacks, including relatively poor resolution insofar as the ability of such a device to determine the location of a subsurface discontinuity and relatively poor sensitivity with respect to an ability to detect small subsurface discontinuities such as short hairline cracks that lie a significant distance below the surface of the object being examined. Both of these problems primarily result from the fact that a relatively low frequency energization signal must be used in order to detect discontinuities that lie relatively deep within the material being inspected, i.e., a substantial distance below the surface thereof. That is, as is known by those skilled in the art, as a magnetic field penetrates a conductive material and establishes eddy currents, the magnetic field is exponentially attenuated at a rate that will decrease the flux density to approximately 37% of its original value at a penetration depth that is customarily identified as the "skin depth". Further, it is known that such skin depth is inversely proportional to both the square root of the frequency at which the field varies and the square root of both the material conductivity and permeability. Accordingly, prior art CW detection systems require a low frequency excitation signal in order to establish eddy currents within portions of the material that lie considerably below the surface, especially when materials of relatively high conductivity or permeability are being examined. Although low frequency signals are necessary in order to detect discontinuities that are relatively deep within the material, such low frequency signals are not desirable in that the magnitude of the induced eddy currents is directly proportional to the time rate of change in magnetic flux (Faraday's Law). Thus, although it is necessary to use a low frequency excitation signal in order to effect sufficient penetration of the magnetic field, such low frequency signals do not induce strong eddy current action. Since eddy currents of a relatively high magnitude are necessary so that rather small discontinuities that are relatively deep within the object being inspected will produce easily detectable perturbation or alteration of the magnetic field at the surface of the object being inspected, low frequency CW eddy current inspection devices must operate at relatively high power levels. Even when relatively high power levels are used, presently available CW inspection devices do not exhibit the sensitivity required to detect small discontinuities such as hairline cracks less than 0.5 inches in length that lie on the order of 0.25 inches below the surface of a conductive member.
The use of a low frequency energization signal also affects system resolution in that relatively large coils are generally necessary in order to supply a magnetic field of suitable flux density. In particular, since prior art CW eddy current inspection devices utilize energization currents on the order of several hundred milliamps and operate at relatively high power levels, the drive coils that provide the energizing magnetic field must generally consist of several hundred turns of relatively heavy wire in order to provide a high impedance at the energization frequency while minimizing the resistive component to prevent overheating of the drive coil. Since such coils are relatively large, a magnetic field is supplied to a relatively large region of the conductive material being examined. Thus, although such a system may indicate the presence of a subsurface flaw, the location thereof cannot be accurately determined.
The above-mentioned problems are of special significance in situations in which subsurface flaws are to be detected in conductive material which includes various intended surface features. For example, many structural assemblies such as modern high-speed aircraft employ conductive skin layers that are joined to frame members by the use of various conventional fasteners such as rivets or bolts and are joined together by various splice configurations that employ such fasteners. In maintaining such aircraft and other structures that include multiple layers of conductive material is often necessary and desirable to detect fatique induced cracks or flaws that can develop in one of the subsurface layers, especially along the periphery of the fasteners which join such subsurface layers to other layers of conductive material or various frame members. When prior art CW eddy current inspection techniques are employed in such situations, the relatively large drive coils often induce a magnetic field that encompasses a larger region of the material than is desired. In particular, in many instances the resulting eddy currents will be affected not only by subsurface flaws but by adjoining fasteners, small openings between adjacent pieces of skin material or normal variations in countersunk or counterbored regions of the fastener openings. Thus, oftentimes it is difficult or impossible to determine whether the changes in the magnetic field produced by the induced eddy currents is due to subsurface discontinuities or is due to one of the above mentioned design features.
Various proposals have been made to eliminate or alleviate at least some of the above-discussed disadvantages of prior art CW eddy current inspection devices. For example, U.S. Pat. Nos. 2,965,840 and 3,229,197, issued to C. J. Renken Jr., are illustrative of efforts to utilize periodically pulsed magnetic fields rather than sinusoidally varying fields to induce eddy currents in conductive materials. Such pulse techniques minimize power handling requirements and hence reduce the size of the drive coils to improve resolution. Further, in the apparatus disclosed in the Renken patents, a drive coil is excited with a pulse signal wherein pulses of relatively long-time duration are supplied in alternation with pulses of relatively short-time duration to thereby successively supply magnetic fields which deeply penetrate the material being examined and magnetic fields which penetrate only the surface region of such material. The eddy currents produced by the successive long- and short-duration pulses are then detected and separated from one another and the detected signal that represents the short-duration drive pulse is subtracted from the signal that represents the long-duration drive pulse to supply a signal that is insensitive to spacial variations between the drive coil and the surface of the material being examined. This signal is then utilized to provide an indication of the conductivity of the material being examined and/or subsurface flaws within such material.
Although pulsed techiques such as those disclosed by Renken partially overcome the disadvantages of sinusoidally excited CW eddy current devices, such pulsed techniques still do not provide the sensitivity necessary to detect relatively small cracks or flaws that are located fairly deep within a conductive material. For example, it presently appears that commercially available eddy current devices are incapable of detecting hairline cracks that are less than 0.5 inches in length and lie 0.25 inches or more below the surface of a conductive member. Thus, while prior art eddy current devices may provide satisfactory results in some situations, state-of-the-art limitations prevent application in many important situations. For example, under current design technology, the previously mentioned multilayered aircraft structure can employ a skin layer of 0.25 inches or more in thickness and prior art CW techniques are not satisfactory in detecting fatigue induced cracks or other flaws within the second layer of material. Further, although the pulsed technique disclosed in the Renken patents presumably eliminates a portion of the eddy currents produced at the surface of the material being inspected, such a technique has not proven totally satisfactory in detecting subsurface flaws in the presence of various intended design features. For example and as previously mentioned, when multilayered structure is joined together or joined to other structural members, adjacent edges of the surface panels are often separated by a small gap and the openings for the fasteners are often counterbored or countersunk. Since such boundary edges and fastener features extend entirely through or substantially into the top layer of a multiple layered structure, substantial perturbations in the eddy currents can be produced which, under prior art techniques, cannot be discerned from unintended subsurface flaws.
In addition, the pulsed techniques of the Renken patents and various other approaches that have been utilized to improve performance of prior art eddy current detection devices require rather complicated apparatus for producing signals indicative of the various components of the composite magnetic field and, in effect, separating the eddy currents generated within the conductive material from either the original, energizing signal or the eddy currents generated at the surface of the material. For example, the apparatus disclosed in both of the previously mentioned Renken patents includes rather complex gating circuitry, signal separation and stabilization circuitry and signal compensation circuitry.
Accordingly it is an object of this invention to provide a method and apparatus for inducing eddy currents within conductive structure to determine subsurface flaws that lie deep within a material being examined.
It is another object of this invention to provide a method and apparatus for eddy current detection of subsurface flaws wherein the resolution achieved permits detection of relatively small discontinuities such as relatively short hairline cracks that lie below the surface of the material being examined.
It is yet another object of this invention to provide a method and apparatus for eddy current detection of relatively small subsurface cracks and flaws that are located about the periphery of fasteners that join the adjacent layers of a multilayered structure or form splices between adjacent surface panels.
Still further, it is an object of this invention to provide a method and apparatus of the above-described type wherein subsurface flaws, including those within the interior layers of a multilayered structure, are detectable in the presence of various surface irregularities and structural features.
Even further, it is an object of this invention to provide eddy current inspection apparatus of the above-described type which does not require relatively complex signal processing arrangements and, hence, can be easily and economically produced.